1. Field of the Invention
The present invention relates to an electric power steering system which assists a steering operation by applying a driving force to a steering mechanism from an electric motor controlled to be driven according to a steering torque.
2. Description of Related Art
Conventionally, motor vehicles incorporate an electric power steering system which assists a steering operation by transmitting a driving force to a steering mechanism from an electric motor. The electric motor is feedback-controlled on the basis of a target electric current determined in accordance with a steering torque applied to a steering wheel, whereby a steering assist force is applied to the steering mechanism in accordance with the steering torque.
When a motor vehicle travels a long and gently curved road of an entrance ramp, an exit ramp or a main lane on a highway, or turns to change its traveling direction, the motor vehicle assumes a turning state to turn in a circular orbit of a generally constant curvature. When the motor vehicle is in the turning state, a driver applies a generally constant torque to the steering wheel to keep the steering wheel at a generally constant steering angle, i.e., the driver performs a steering hold-on operation.
When the motor vehicle is in the turning state mentioned above, the driver should perform the steering hold-on operation for a relatively long period. Even with the steering assist, the steering hold-on operation is burdensome to the driver and, therefore, leaves room for improvement to provide a better steering feeling.
It is an object of the present invention to provide an electric power steering system which alleviates the burden to a driver for the improvement of the steering feeling when a motor vehicle is in the turning state.
An electric power steering system according to the present invention, which assists a steering operation by controlling an electric motor on the basis of a target electric current determined in accordance with a steering torque applied to an operation member and applying a driving force to a steering mechanism from the electric motor, comprises: a steering torque detecting section for detecting a steering torque; a target electric current determining circuit for determining a target electric current in accordance with the steering torque detected by the steering torque detecting section on the basis of an assist characteristic indicative of a relationship between the steering torque and the target electric current; a motor controlling circuit for controlling the electric motor on the basis of the target electric current determined by the target electric current determining circuit for driving the electric motor; and an assist characteristic modifying circuit for modifying the assist characteristic by offsetting the assist characteristic from an initial characteristic when a motor vehicle incorporating the electric power steering system is in a predetermined turning state.
In accordance with the present invention, the assist characteristic which determines the target electric current according to the steering torque is offset from the initial characteristic for the modification thereof when the motor vehicle is in the turning state. Thus, a steering assist force to be generated in the turning state has a magnitude different from that to be generated in a traveling state other than the turning state.
More specifically, if the assist characteristic is offset from the initial characteristic so as to cause the electric motor to generate a greater driving force in the turning state, a steering assist force greater than that to be applied in the other traveling state can be applied to the steering mechanism. Thus, a steering hold-on operation can properly be assisted, so that the burden to the driver can be alleviated for the improvement of the steering feeling.
In the electric power steering system, the assist characteristic modifying circuit may comprise a circuit for determining that the motor vehicle is in the predetermined turning state on condition that the steering torque detecting section keeps detecting a torque not smaller than a predetermined level for not less than a predetermined period.
The electric power steering system may further comprise a steering angle detecting section for detecting a steering angle of the operation member. In this case, the assist characteristic modifying circuit preferably comprises a circuit for determining that the motor vehicle is in the predetermined turning state on condition that the steering angle detecting section keeps detecting a steering angle not smaller than a predetermined level for not less than a predetermined period.
The electric power steering system may further comprise a vehicle speed detecting circuit for detecting a speed of the motor vehicle. In this case, the assist characteristic modifying circuit preferably offsets the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not higher than a first vehicle speed.
With this arrangement, the assist characteristic is offset from the initial characteristic on condition that the vehicle speed is not higher than the first vehicle speed. This arrangement is preferably applied, for example, to a case where the motor vehicle turns to change its traveling direction. Where the first vehicle speed is set at about 30 km/hour, for example, the assist characteristic may be offset when the motor vehicle is in a turning state with a relatively great steering angle (a first steering angle which may be, for example, a rotation angle of a steering wheel of not smaller than about 180 degrees). Thus, a relatively great steering assist force can be applied to the steering mechanism in the turning state, so that the burden to the driver can be alleviated for the improvement of the steering feeling.
Where the electric power steering system further comprises the vehicle speed detecting section for detecting the speed of the motor vehicle, the assist characteristic modifying circuit may offset the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not lower than a second vehicle speed.
The second vehicle speed is preferably higher than the first vehicle speed.
Where the assist characteristic is gradually changed from the initial characteristic to the offset characteristic, the rate of the change in the assist characteristic is preferably lower in a case where the vehicle speed is not lower than the second vehicle speed than in a case where the vehicle speed is not higher than the first vehicle speed.
With this arrangement, the assist characteristic is offset from the initial characteristic on condition that the vehicle speed is not lower than the second vehicle speed. This arrangement is preferably applied to a case where the motor vehicle travels a gently curved road on a highway. Where the second vehicle speed is set at about 80 km/hour, for example, the assist characteristic may be offset when the motor vehicle is in a turning state with a relatively small steering angle (a second steering angle which is, for example, a rotation angle of the steering wheel of not smaller than about 15 degrees). Thus, the steering assist force can be increased when the motor vehicle travels the gently curved road on the highway, so that the burden to the driver for the steering hold-on operation can be alleviated for the improvement of the steering feeling.
The assist characteristic modifying circuit preferably comprises an assist characteristic transition circuit for gradually effecting transition of the assist characteristic to an initial offset characteristic which is offset by a predetermined value from the initial characteristic when the motor vehicle comes into the predetermined turning state.
With this arrangement, the transition of the assist characteristic from the initial characteristic to the initial offset characteristic is gradually effected, so that an abrupt change in the steering assist force can be prevented for further improvement of the steering feeling.
The assist characteristic modifying circuit preferably further comprises a torque adapting circuit for further modifying the assist characteristic in accordance with the change in the steering torque detected by the steering torque detecting section after the transition of the assist characteristic to the initial offset characteristic.
With this arrangement, the assist characteristic is further modified in accordance with the change in the steering torque after the transition of the assist characteristic to the initial offset characteristic. Thus, the assist characteristic can be adapted for the steering state, thereby further improving the steering feeling.
Where the steering wheel is operated to increase the steering angle in the turning state, for example, the steering torque is increased, so that an assist torque is increased. Where the steering wheel is returned, on the contrary, the steering torque is reduced, so that the assist torque is reduced. Therefore, a difference between the steering torques applied to the steering wheel in the opposite directions for the correction of the steering operation is reduced, so that the burden for the steering operation is reduced.
The assist characteristic modifying circuit preferably further comprises an initial offset determining circuit for determining the initial offset characteristic in accordance with a magnitude of the steering assist force applied during a predetermined period before the assist characteristic is changed to the initial offset characteristic.
With this arrangement, the initial offset characteristic is determined on the basis of the magnitude of the steering assist force (the magnitude of the steering torque or the level of the target electric current) before the assist characteristic is changed. Therefore, the initial offset characteristic can be optimized for further improvement of the steering feeling.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.